1. Field of the Invention
This invention generally relates to a method for the production of graphene, and more specifically to an improved method for synthesizing micrometer scale graphene sheets by, in one embodiment, the simultaneous vacuum thermal decomposition of two SiC substrates placed in close, face to face proximity to each other.
2. Background Prior Art
Graphene, a two-dimensional array of carbon atoms in a honeycomb lattice, has been theoretically studied for decades in terms of the fundamental building block of carbon based materials, such as graphite and carbon nanotubes [1, 2]. However, it had not been believed to exist due to thermodynamic instability until Novoselov et al. succeeded in isolating single graphene layers from graphite by means of micromechanical cleavage in 2004 [3]. The realization of graphene has stimulated a large amount of experimental research which led to the discovery of numerous novel properties, including quantum Hall effects, and relativistic quasi particles with a group velocity of 1/300 c (where c is the speed of light) [4-6]. High values of conductance, mobility, and mechanical strength have also been observed [4, 7-8]. Based on these properties, graphene shows promising potentials for a wide variety of new technological applications [9-11], such as post-CMOS digital electronics, single-molecule gas sensors, spintronic devices, etc.
In order for graphene to be successfully applied to new devices, homogeneous growth of graphene with device-sized scale (micrometer) on a semiconducting or insulating substrate is essential. Several approaches, such as mechanical exfoliation of graphite followed by transportation onto the surface of SiO2 or mica [3, 12] and epitaxial growth on an SiC substrate in vacuum [13] have been proposed to meet this goal. Although mechanical exfoliation has yielded results, large-scale production by this method is intrinsically limited. Since the first report by Berger et al. [9], epitaxial growth by thermal decomposition of SiC has been considered a viable route for fabricating large area graphene. However, poorly controlled (excessively rapid) silicon sublimation rates from SiC substrate leads to poor quality of graphene as the carbon atoms are unable to self-arrange properly before too much silicon sublimates from below. Externally applied Si supplements, such as disilane (Si2H6), has been used to compensate for the rate of Si sublimation to alleviate this problem [14, 15]. Alternatively, the sublimation rate has been slowed down by epitaxial growth under the ambient pressures of noble gas [16, 17]. Both methods allow for better annealing conditions to produce more homogenous graphene layers.
The two known methods for compensating Si sublimation during graphene growth pose technical challenges. Disilane is a pyrophoric gas (it can spontaneously burst into flame when in contact with air), requiring costly safety precautions. Graphene growth under ambient pressures of noble gases requires a sophisticated furnace. The method described here is much more easily realized than these two known methods in that (1) no hazardous materials are involved, and (2) very simple arrangements can be used for heating the SiC substrates, for example direct-current heating.